1. Field of the Invention
The present invention relates to a method for controlling an early casting stage, from the start of pouring molten steel to the start of drawing a dummy bar, in a continuous casting process.
2. Description of the Related Art
It is well known that a continuous casting process is carried out by holding molten steel supplied by a ladle or the like in a tundish and then pouring the molten steel into a mold from the tundish through an immersion nozzle. The immersion nozzle is usually provided with a flow rate controlling apparatus such as a sliding nozzle or the like.
Since the continuous casting mold is opened at the top and the bottom, the mold is first provided with the head of a dummy bar (hereafter referred to as dummy bar head) at the start of the casting process, the bottom of the mold is closed, and the molten steel is then poured into the mold. Cooling of the molten steel poured into the mold starts at the surface brought into contact with the mold wall, and accordingly, solidified shells are sequentially formed.
When the solidified shells reach a desired thickness, and at the same time the molten steel level in the mold reaches a predetermined level, a dummy bar is drawn. The time from the start of the pouring of the molten steel into a mold to the start of the drawing of the dummy bar is defined as the molten steel holding time in a mold (hereinafter referred to as the holding time).
A very short holding time will cause a breakout to occur, in which the solidified shells are broken by a drawing force of a strand due to an insufficient formation of the solidified shells, and thus the continuous casting process must be stopped. On the other hand, a very long holding time will cause seizing to occur between a solidified shell and the dummy bar head, and accordingly, separation of the two becomes difficult. Since damage generated during the very short holding time is remarkably larger than that generated during the very long holding time, conventional control at an early casting stage is carried out by determining the timing of the start of the drawing so as to ensure a necessary holding time, predetermined with reference to past experience, as a first condition.
As disclosed in Japanese Unexamined Patent Publication No. 58-84652 a continuous casting technique is proposed, wherein an amount of molten steel and the degrees of opening of the sliding nozzle corresponding thereto are calculated from moment to moment from the depth of the molten steel in a tundish, with reference to the molten steel bath level rising pattern (below bath rising patterns) in a mold in which the bath level rising pattern is predetermined by attaining a proper holding time, and control of an amount of molten steel poured is carried out in accordance with this calculation. In an actual operation, however, the flow velocity and flow rate of molten steel poured into a mold are easily changed by variations in the nozzle characteristics, and other problems that arise such as an incorrect depth, temperature, and composition of the molten metal in a tundish, or an unsatisfactory operation of the nozzle.
Thus, in the former process, the process control can not follow charges in the amount of molten steel poured and the drawing process is often started in a state such that the molten steel level is not within a suitable range, as explained below. Further, in the latter process, since the moment-to-moment molten steel level is not compared with the predetermined bath level rising pattern, the molten steel is poured as it is even if the flow velocity of the poured molten steel does not correspond to the predetermined velocity. Therefore, the proper holding time cannot be attained, or the drawing process is commenced after the holding time is finished.
The above-mentioned conventional process comprises a step of controlling the pouring of the molten steel without considering an actual flow velocity thereof, namely, controlling the rising speed of the bath level in the mold. Thus, it is difficult to maintain a constant holding time because of various malfunctions in the process. Consequently, a breakout will occur and a shift to bath level control in a usual operation, cannot be smoothly carried out.